1. Field of the Invention
The present invention generally relates an auto focus control apparatus for a camera module, and more particularly to an auto focus control apparatus for a camera module which can provide excellent preview image quality prior to shooting and/or editing an image after acquisition.
2. Description of the Related Art
Camera modules for performing image recognition functions in devices, such as, for example, a video camera, an electronic still camera, a PC camera terminal, a camera phone, and the like are known in the art.
As the size of camera modules and camera screens have become increasingly miniaturized with the development of high density technology, various miniaturized camera modules are now available to allow users to manually photograph subjects without support equipment such as tripods. Moreover, miniaturization has enabled the camera modules to be mounted directly to video phones, thereby providing a user with the capability of taking photographs while speaking on the phone.
As is well known in the art, a camera module forms an image on an image sensor surface which is a component of a lens optical system. The characteristics of a lens optical system are such that images are transferred to the image surface in a reliable manner without distortion.
The position of the image surface of a lens optical system is changed according to the distance between a lens and a subject. A high quality picture can only be obtained when the degree of variation in the position of the image surface relative to the position of a subject is within the focal range of the camera. However, even a camera module having a deep focal range cannot focus on multiple subjects at staggered distances from the camera module.
Therefore, an auto focus control apparatus capable of controlling the relative position of a lens and a sensor according to the distance between the camera module and a subject is provided in the camera module which can photograph the subject at a short distance and includes a micro function for photographing an image without distortion.
Auto focus control apparatuses of camera modules include actuators capable of changing the position of lenses and sensors. Driving methods of such auto focus control apparatuses includes the use of a stepping motor, a voice coil motor, a piezoelectric motor, and the like.
The stepping motor method and the voice coil motor method have been used in devices such as digital cameras and camera phones. The voice coil motor method is primarily used in a camera phone requiring only an auto focus control apparatus function without an optical zoom function.
The voice coil motor method uses Lorentz forces generated by inductive magnetic forces of coils in magnetic fields formed by permanent magnets for linear movement, and is suitable for precise linear movement over a short distance.
FIG. 1 illustrates a conventional auto focus control apparatus 1 of a camera module which uses the voice coil motor method. As shown in FIGS. 1-3, a cylindrical lens housing 2 is shown extended in a lengthwise direction. A lens barrel 3 is shown mounted in the direction of an optical axis A1. A plurality of focus control lenses 3a are shown mounted along the optical axis A1. A coil 4 is wound along the circumference of the lens barrel 3 in the direction of the optical axis A1. A yoke 5 to which permanent magnets 5a are mounted, to interact with the coil 4, is provided on the outer circumferential side of the lens barrel 3. Resilient members 6 are provided on the lower side of the lens barrel 3 for providing resilient forces so that the lens barrel 3 can be moved, in a linear fashion, in the direction of the optical axis A1. In FIG. 2, the lens housing 2 is shown mounted to the upper portion of a sensor assembly 7.
Referring to FIG. 2, the sensor assembly 7 includes a printed circuit board 7a, an image sensor 7b which is a pickup device attached to the printed circuit board 7a, and an infrared filter 7c provided above the image sensor 7b. The lower end surface of the printed circuit board 7a is electrically connected to a flexible printed circuit board (FPCB).
As shown in FIGS. 2 and 3, when an exciting current is applied to the coil 4 of the lens barrel 3, an electromagnetic force is generated in the magnetic fields of the permanent magnets 5a, causing the lens barrel 3 to move upward and downward, in a linear fashion, in the direction of the optical axis A1. The electromagnetic force is proportional to the strength of the applied current. Therefore, by regulating the strength of the applied current, the position of the lens barrel 3 is regulated to control the focus of the lenses 3a. 
The lens barrel 3 is engaged with the lens housing 2 to regulate the focus of the lenses 3a during the process of assembling the lens barrel 3 to the lens housing 2. The focus height is regulated by engaging the lens housing 2 with the sensor assembly 7 by rotating the lens housing 2 to regular the focus of the lenses during the process of assembling the lens housing 2 in which the lens barrel 3 is installed to the sensor assembly 7.
One drawback associated with the conventional voice coil motor method, described above, is that since the auto focus control apparatus regulates the focus by rotating a screw to regulate the lens focus height initially, the size of the camera module increases. A further drawback is the time required to regulate the focus initially, primarily due the fact that the focus height cannot be regulated precisely. That is, since the focus height is regulated manually by an operator, it is difficult to regulate the focus height accurately. In this state, the lens housing 2 is engaged with an upper portion of the sensor assembly 7 by a screw rotation portion. Furthermore, in the case where the focus is not regulated properly by removing the screw rotating portion, the quality of the preview image deteriorates.
A further drawback is the inability to miniaturize the camera module due to the conventional coil being wound on the outer circumferential side of the lens barrel in the optical axis direction, thereby increasing the thickness of the camera module.